1. Field of the Invention
The present invention relates to semiconductor light emitting devices, and more particularly to a semiconductor light emitting device having as a light emitting layer a gallium nitride base compound semiconductor layer expressed by a general formula of InxGayAlzN that is provided on a substrate having a smaller coefficient of thermal expansion than GaN, with an intermediate layer interposed therebetween.
2. Description of the Background Art
Of the nitride semiconductor material systems employing GaN, InN, AlN and their mixed crystal semiconductors, a semiconductor light emitting device using InxGa1-xN crystal as a light emitting layer has conventionally been fabricated employing a sapphire substrate as its substrate primarily.
When a Si substrate is applied to the material system as the substrate, it will be possible to fabricate a less expensive semiconductor light emitting device, because the Si substrate is less expensive than the sapphire substrate and the one having a large area is commercially available.
Here, as an attempt to crystal grow a nitride semiconductor film on the Si substrate, providing a BAlGaInN base single-layer or multi-layer structure as an intermediate layer to fabricate a nitride base semiconductor light emitting device has been disclosed in Japanese Patent Laying-Open Nos. 5-343741 and 2000-277441.
Further, the following publication 1 describes a way of fabricating a nitride base semiconductor light emitting device by stacking an AlN layer and an Al0.27Ga0.73N layer one another for use as an intermediate layer.
Publication 1: M. Adachi et al., xe2x80x9cFabrication of Light Emitting Diodes with GaInN Multi-Quantum Wells on Si(111) Substrate by MOCVDxe2x80x9d, Proc. Int. Workshop on Nitride Semiconductors, IPAP Conf Series 1, pp. 868-871.
For the combination technique for performing lattice alignment, however, adequate studies have yet to be made. Based on the results of the inventors"" studies, when a substrate such as a Si substrate having a smaller coefficient of thermal expansion than a nitride semiconductor film is employed, it would be difficult to grow a nitride semiconductor film of good quality and less dislocation by simply providing such an intermediate layer as described in the above publication. A light emitting layer fabricated on the film would suffer considerable dislocation, hindering implementation of a high-luminance light emitting device.
Further, when a nitride base semiconductor device is fabricated on a Si substrate, cracks would occur due to the difference in coefficient of thermal expansion when the fabricated film is cooled to room temperature. Thus, it has been found that it is important to employ hard AlN to reduce occurrence of such cracks.
In other words, when a substrate having a lattice constant different from that of and a coefficient of thermal expansion smaller than that of a nitride semiconductor film is being employed, it is necessary to grow an AlGaInN layer containing a large amount of AlN exhibiting high degrees of c-axis orientation and hardness. This AlGaInN intermediate layer, however, has a low lattice constant, due to AlN contained in such a large amount, and would apply large compressive strain to a GaInN light emitting layer constituting the light emitting device structural portion, thereby deteriorating its crystallinity and degrading the luminous efficiency.
For example, in the structure described in the above publication 1, an intermediate layer 102 is formed of an AlN layer 102a of a thickness of 120 nm (a-axis lattice constant: 0.3112 nm) and an Al0.27Ga0.73N layer 102b of a thickness of 380 nm (a-axis lattice constant: 0.3168 nm) stacked one another on a Si substrate 101, as shown in FIGS. 5A and 5B. On the intermediate layer 102, a GaN layer 103 (a-axis lattice constant: 0.3189 nm) and a GaInN light emitting layer 106 are formed.
The lattice constant described herein is simply an a-axis lattice constant of a bulk, i.e., one theoretically calculated using the Vegard""s Law, because an actual lattice constant would change according to deformation such as strain, thereby introducing discrepancies to the values.
FIGS. 5A and 5B respectively show a schematic cross section of the configuration of the semiconductor light emitting device described in the above publication 1 and the a-axis lattice constants of the respective layers in the bulk states.
As such, the lattice constant of AlGaInN base intermediate layer 102 can be increased by lowering the content of Al or increasing the content of Ga or In therein gradually or stepwise. The AlGaInN intermediate layer 102 of multi-layer structure thus permits lattice alignment from Si substrate 101 to GaN layer 103. Such an lattice alignment effect, however, is insufficient with only the multi-layer AlGaInN intermediate layer 102. Dislocation is obvious on this intermediate layer 102, making it difficult to grow GaN layer 103 of good quality. As such, when a light emitting layer 106 is formed on GaN layer 103 and a voltage is applied thereto, an unproductive leakage current not contributing to the emission of light emitting layer 106 would increase, hindering implementation of a high-luminance semiconductor light emitting device.
An object of the present invention is to provide a long-life and high-luminance nitride base semiconductor light emitting device, when a substrate such as a Si substrate having a smaller coefficient of thermal expansion than a nitride semiconductor film is employed, by suppressing occurrence of cracks and ensuring good crystallinity of the nitride semiconductor film.
The semiconductor light emitting device of the present invention is a semiconductor light emitting device having a gallium nitride base compound semiconductor layer expressed by a general formula of InxGayAlzN (x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), characterized in that it includes one intermediate layer between a first GaN layer and a light emitting layer and that the one intermediate layer has a lattice constant that is closer to a lattice constant of the light emitting layer than a lattice constant of the first GaN layer.
According to the semiconductor light emitting device of the present invention, provision of the one intermediate layer having a lattice constant closer to that of the light emitting layer than that of the first GaN layer permits sufficient lattice alignment, and thus effectively reduces strain applied to the light emitting layer. A first GaN layer of high quality, suppressed with occurrence of dislocation, can be obtained. Accordingly, it is possible to obtain a long-life and high-luminance semiconductor light emitting device.
Preferably, the semiconductor light emitting device described above is further provided with a substrate having a smaller coefficient of thermal expansion than GaN, and another intermediate layer formed between the substrate and the first GaN layer. The another intermediate layer has a lattice constant that is closer to the lattice constant of the first GaN layer than a lattice constant of the substrate.
The another intermediate layer permits lattice alignment between the substrate and the first GaN layer. Accordingly, it is possible to obtain a first GaN layer of high quality with occurrence of dislocation being suppressed.
Preferably, in the semiconductor light emitting device described above, the another intermediate layer includes an AlaGabIn1-a-bN layer (0xe2x89xa6axe2x89xa61, 0xe2x89xa6dxe2x89xa61, a+bxe2x89xa61).
Including the hard AlN layer in the another intermediate layer of AlaGabIn1-a-bN layer prevents occurrence of cracks due to the difference in coefficient of thermal expansion.
Preferably, in the semiconductor light emitting device described above, the one intermediate layer includes an INcGadAl1-c-dN layer (0 less than cxe2x89xa61, 0xe2x89xa6dxe2x89xa61, c+dxe2x89xa61).
This allows the lattice constant of the one intermediate layer to come closer to that of the light emitting layer than that of the first GaN layer.
Preferably, in the semiconductor light emitting device described above, the AlaGabIn1-a-bN layer has a plurality of layers of an AloGafIn1-e-fN layer (0 less than exe2x89xa61, 0xe2x89xa6fxe2x89xa61, e+fxe2x89xa61) and an AlgGahIn1-g-hN layer (0 less than gxe2x89xa61, 0xe2x89xa6hxe2x89xa61, g+hxe2x89xa61, e less than g) sequentially stacked, and a lattice constant of the AleGafIn1-e-fN layer is smaller than that of the first GaN layer.
This assures high hardness and thus prevents cracks due to the difference in coefficient of thermal expansion. In addition, the lattice constant of the AlgGahIn1-g-hN layer, rather than that of the AleGafIn1-e-fN layer, can be made closer to that of the GaN layer, so that a still further lattice alignment effect can be obtained.
Preferably, in the semiconductor light emitting device described above, the AlaGabIn1-a-bN layer consists of a plurality of layers, and the plurality of layers each have a smaller Al composition ratio as it is closer to the first GaN layer.
Thus, high hardness is obtained, preventing cracks due to the difference in coefficient of thermal expansion. The lattice alignment effect also increases.
Preferably, the semiconductor light emitting device described above is further provided with a clad layer formed between the one intermediate layer and the light emitting layer. The clad layer includes at least one of a second GaN layer and an IniGa1-iN layer (0 less than ixe2x89xa61).
Forming the clad layer on the surface of the one intermediate layer poor in flatness improves the surface flatness, thereby preventing generation of a leakage current. In addition, a carrier block effect is obtained by forming the clad layer.
Preferably, in the semiconductor light emitting device described above, the clad layer has a film thickness of not less than 10 nm and not more than 30 nm.
This provides the effect of improving the surface flatness.
If the clad layer is thinner than 10 nm, the effect of improving the surface flatness would not be expected sufficiently. If it is thicker than 30 nm, strain on the intermediate layer would be recovered by the GaN layer, which adversely affects the quality of the light emitting layer.
Preferably, in the semiconductor light emitting device described above, the IncGadAl1-c-dN layer consists of a plurality of layers, and the plurality of layers each have a smaller In composition ratio as it is closer to the light emitting layer.
This assures a more remarkable lattice alignment effect.
Preferably, in the semiconductor light emitting device described above, the AlaGabIn1-a-bN layer has a film thickness of not less than 10 nm and not more than 500 nm.
If the AlaGabIn1-a-bN layer is thinner than 10 nm, the c-axis orientation of the clad layer would be degraded, which makes the crystal coarse, hindering implementation of a high-luminance semiconductor light emitting device. If the AlaGabIn1-a-bN layer is thicker than 500 nm, although the lattice constant change might be modest, the total thickness of the light emitting device structure would increase. As a result, strain attributable to the difference in coefficient of thermal expansion between the substrate and the GaN layer would increase, and cracks would also occur. This leads to an increase of the leakage current of the semiconductor light emitting device, making it difficult to fabricate a high-luminance semiconductor light emitting device.
Preferably, in the semiconductor light emitting device described above, the IncGadAl1-c-dN layer has a film thickness of not less than 200 nm and not more than 400 nm.
If the IncGadAl1-c-dN layer is thinner than 200 nm, a sufficient lattice alignment effect would not be expected. This lessens the effect of decreasing the strain, thereby hindering implementation of a high-luminance semiconductor light emitting device.
If the IncGadAl1-e-dN layer is thicker than 400 nm, the total thickness of the light emitting device structure on the substrate would increase. Cracks would occur, and the leakage current of the semiconductor light emitting device would increase, as described above, making it difficult to fabricate a high-luminance semiconductor light emitting device.
Preferably, in the semiconductor light emitting device described above, a ratio of the In content to the Ga content in the IncGadAl1-c-dN layer is not more than 10%.